Using The Level-Ack Protocol - National Instruments 653 Series User Manual

Using the Level-ACK Protocol

ACK
REQ
Reference
Point
Initial State ACK is deasserted. The NI 653X waits for an active REQ to indicate that the
peripheral device is ready. The peripheral device may optionally drive the first
data at this time. The transfer cannot begin until the peripheral asserts REQ; the
peripheral may either pulse REQ or hold REQ high until the first ACK occurs.
If the peripheral pulses REQ, make sure to start the transfer on the NI 653X
before the pulse occurs to avoid missing the pulse.
1 The NI 653X waits until it has space for data, then it asserts ACK.
2 The peripheral device can then strobe data into the NI 653X by first deasserting
then asserting the REQ signal. The NI 653X waits for an active-going transition
on the REQ line. ACK stays asserted, indicating the NI 653X is ready, until the
active-going REQ occurs.
3 The active-going REQ signal edge deasserts the ACK signal and causes the
NI 653X to latch input data.
4 To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
© National Instruments Corporation
In level-ACK protocol, the NI 653X asserts the ACK signal when ready for
a transfer and holds the ACK signal level until an active-going edge occurs
on the REQ line. After the REQ edge occurs, the NI 653X deasserts the
ACK signal until the device is ready for another transfer.
1
2
Initial State
ACK and REQ are shown as active high.
Steps 1-4 are repeated for each transfer.
Figure 3-14. Level-ACK Input Handshaking Sequence
3
4
Action Steps
3-17
Chapter 3 Timing Diagrams
NI 653X User Manual